Pierce oscillators are a commonly used type of oscillator circuit. For instance, a Pierce oscillator circuit can include a single inverting amplifier, a crystal, and two capacitors, where the crystal acts as a highly frequency dependent reactance element. The crystal and the two capacitors form a very high Q tank circuit, which provides a phase shift and gain from an output of the inverting amplifier to an input of the inverting amplifier at approximately the resonant frequency of the crystal. The crystal operates inductive at the frequency of oscillation, where an impedance is determined by the passive elements; thus, the operating impedance is >+j0Ω. For instance, the operating impedance of the crystal can approximately equal to a sum of the magnitudes of the impedances of the two capacitors.
In conventional Pierce oscillators, the inverting nature of the signals across the crystal can cause significant drive voltages across the crystal. However, the crystal may be sensitive to the drive voltage. Accordingly, accuracy of these conventional oscillator circuits can be detrimentally impacted due to relatively high drive levels across the crystal. Moreover, some crystals may be damaged if operated at these relative high drive levels.